149 lines
6.0 KiB
Markdown
149 lines
6.0 KiB
Markdown
LZ4 Block Format Description
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============================
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Last revised: 2018-04-25.
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Author : Yann Collet
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This specification is intended for developers
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willing to produce LZ4-compatible compressed data blocks
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using any programming language.
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LZ4 is an LZ77-type compressor with a fixed, byte-oriented encoding.
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There is no entropy encoder back-end nor framing layer.
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The latter is assumed to be handled by other parts of the system (see [LZ4 Frame format]).
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This design is assumed to favor simplicity and speed.
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It helps later on for optimizations, compactness, and features.
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This document describes only the block format,
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not how the compressor nor decompressor actually work.
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The correctness of the decompressor should not depend
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on implementation details of the compressor, and vice versa.
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[LZ4 Frame format]: lz4_Frame_format.md
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Compressed block format
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-----------------------
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An LZ4 compressed block is composed of sequences.
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A sequence is a suite of literals (not-compressed bytes),
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followed by a match copy.
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Each sequence starts with a `token`.
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The `token` is a one byte value, separated into two 4-bits fields.
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Therefore each field ranges from 0 to 15.
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The first field uses the 4 high-bits of the token.
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It provides the length of literals to follow.
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If the field value is 0, then there is no literal.
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If it is 15, then we need to add some more bytes to indicate the full length.
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Each additional byte then represent a value from 0 to 255,
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which is added to the previous value to produce a total length.
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When the byte value is 255, another byte is output.
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There can be any number of bytes following `token`. There is no "size limit".
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(Side note : this is why a not-compressible input block is expanded by 0.4%).
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Example 1 : A literal length of 48 will be represented as :
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- 15 : value for the 4-bits High field
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- 33 : (=48-15) remaining length to reach 48
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Example 2 : A literal length of 280 will be represented as :
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- 15 : value for the 4-bits High field
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- 255 : following byte is maxed, since 280-15 >= 255
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- 10 : (=280 - 15 - 255) ) remaining length to reach 280
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Example 3 : A literal length of 15 will be represented as :
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- 15 : value for the 4-bits High field
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- 0 : (=15-15) yes, the zero must be output
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Following `token` and optional length bytes, are the literals themselves.
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They are exactly as numerous as previously decoded (length of literals).
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It's possible that there are zero literal.
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Following the literals is the match copy operation.
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It starts by the `offset`.
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This is a 2 bytes value, in little endian format
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(the 1st byte is the "low" byte, the 2nd one is the "high" byte).
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The `offset` represents the position of the match to be copied from.
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1 means "current position - 1 byte".
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The maximum `offset` value is 65535, 65536 cannot be coded.
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Note that 0 is an invalid value, not used.
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Then we need to extract the `matchlength`.
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For this, we use the second token field, the low 4-bits.
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Value, obviously, ranges from 0 to 15.
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However here, 0 means that the copy operation will be minimal.
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The minimum length of a match, called `minmatch`, is 4.
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As a consequence, a 0 value means 4 bytes, and a value of 15 means 19+ bytes.
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Similar to literal length, on reaching the highest possible value (15),
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we output additional bytes, one at a time, with values ranging from 0 to 255.
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They are added to total to provide the final match length.
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A 255 value means there is another byte to read and add.
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There is no limit to the number of optional bytes that can be output this way.
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(This points towards a maximum achievable compression ratio of about 250).
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Decoding the `matchlength` reaches the end of current sequence.
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Next byte will be the start of another sequence.
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But before moving to next sequence,
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it's time to use the decoded match position and length.
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The decoder copies `matchlength` bytes from match position to current position.
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In some cases, `matchlength` is larger than `offset`.
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Therefore, `match_pos + matchlength > current_pos`,
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which means that later bytes to copy are not yet decoded.
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This is called an "overlap match", and must be handled with special care.
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A common case is an offset of 1,
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meaning the last byte is repeated `matchlength` times.
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Parsing restrictions
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-----------------------
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There are specific parsing rules to respect in order to remain compatible
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with assumptions made by the decoder :
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1. The last 5 bytes are always literals. In other words, the last five bytes
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from the uncompressed input (or all bytes, if the input has less than five
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bytes) must be encoded as literals on behalf of the last sequence.
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The last sequence is incomplete, and stops right after the literals.
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2. The last match must start at least 12 bytes before end of block.
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The last match is part of the penultimate sequence,
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since the last sequence stops right after literals.
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Note that, as a consequence, blocks < 13 bytes cannot be compressed.
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These rules are in place to ensure that the decoder
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can speculatively execute copy instructions
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without ever reading nor writing beyond provided I/O buffers.
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1. To copy literals from a non-last sequence, an 8-byte copy instruction
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can always be safely issued (without reading past the input),
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because literals are followed by a 2-byte offset,
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and last sequence is at least 1+5 bytes long.
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2. Similarly, a match operation can speculatively copy up to 12 bytes
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while remaining within output buffer boundaries.
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Empty inputs can be represented with a zero byte,
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interpreted as a token without literals and without a match.
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Additional notes
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-----------------------
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There is no assumption nor limits to the way the compressor
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searches and selects matches within the source data block.
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It could be a fast scan, a multi-probe, a full search using BST,
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standard hash chains or MMC, well whatever.
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Advanced parsing strategies can also be implemented, such as lazy match,
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or full optimal parsing.
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All these trade-off offer distinctive speed/memory/compression advantages.
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Whatever the method used by the compressor, its result will be decodable
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by any LZ4 decoder if it follows the format specification described above.
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